The present invention is described for an optical disc drive. However, the present invention may in general be used for calibration within other types of disc drives.
FIG. 1 shows a block diagram of an optical pick-up 100 of an optical disc drive with light generated at a laser diode 102. The light from the laser diode 102 passes through a grating 104 that splits such light into a main beam and two side beams. The main and side beams of light are reflected by a beam splitter 106 to be directed through a collimating lens 108. The beams of light from the collimating lens 108 are focused by an objective lens 110 onto an optical disc 112.
The beams of light are then reflected from the optical disc 112 and pass back through the objective lens 110 and the collimating lens 108. Such reflected beams of light pass through the beam splitter 106 to reach a second objective lens 114. The second objective lens 114 focuses the reflected beams of light onto a photo-detector 116. A spindle motor mechanism 113 rotates the optical disc 112 during data reproduction.
FIG. 2A shows a magnified view of tracks on the optical disc 112 having a plurality of alternating lands 122 and grooves 124 along the radial direction of the optical disc 112. Further referring to FIG. 2A, data is recorded with data pits (illustrated as blackened areas in FIG. 2A) formed on the grooves 124 in some types of optical discs.
Additionally referring to FIG. 2A, a main beam 126 is directed onto a groove, and first and second side beams 128 and 130 are directed onto the adjacent lands. The first side beam 128 lags the main beam 126, and the second side beam 130 leads the main beam 126. The main and side beams 126, 128, and 130 are formed by the components of the optical pick-up 100 of FIG. 1.
The side beams 128 and 130 are used for tracking to maintain the main beam 126 centered along a groove 124 with data pits. FIG. 2B shows the photo-diode 116 measuring an intensity of the first side beam 128 reflected from the optical disc 112. Referring to FIG. 2B, assume “E” denotes an intensity of light toward a left of the first side beam 128 reflected from the optical disc 112. Similarly, assume “F” denotes an intensity of light toward a right of the first side beam 128 reflected from the optical disc 112. A TE (tracking error) signal is calculated as follows using the E and F signals:TE=E−F 
When the main beam 126 is centered along a groove 124, the value of the TE signal is at a center level, TCNTR. When the main beam 126 and the side beams 128 and 130 are shifted undesirably toward the right, F decreases such that the TE signal changes to be more positive from the center level, TCNTR. When the main beam 126 and the side beams 128 and 130 are shifted undesirably toward the left, E decreases such that the TE signal changes to be more negative from the center level, TCNTR.
For proper tracking operation, the TE signal is desired to be centered about a reference level, VREF. Thus, TCNTR of the TE signal is desired to be substantially equal to the reference level, VREF. Thus, after loading a disc into the disc drive, calibration is performed such that the measured TE signal is calibrated to be centered about the reference level, VREF.
Referring to FIGS. 1 and 3, in the prior art calibration, when the disc 112 is loaded into the disc drive, the disc 112 is rotated, and the TE signal 132 is generated as the optical pick-up 100 is stationary. In FIG. 3, the TE signal 132 has a lower frequency range 134 for a lower rotational speed of the disc 112 and a higher frequency range 136 for a higher rotational speed of the disc 112. The TE signal 132 also has an ideal frequency range 138 for a desired range of rotational speed of the disc 112.
Further referring to FIG. 3, the measured TE signal 132 is centered about the center value, TCNTR, which is different from the reference level, VREF, by an offset 140. During calibration of the disc drive, the amount of offset 140 is determined to adjust the TE signal 132 to be centered about the reference level, VREF.
For such calibration, the TE signal within the ideal frequency range 138, such as 500 Hz to 1000 Hz for example, is used for accurately determining the offset 140. When the TE signal has a higher frequency from the ideal frequency range 138, the amplitude of the TE signal may be too low for accurate determination of the offset 140. When the TE signal has a lower frequency from the ideal frequency range 138, determination of the offset 140 may require too much time.
Unfortunately, such prior art calibration requires a delay until the rotational speed of the optical disc 112 reaches a desired range for the frequency of the TE signal to be within the ideal frequency range 138. In addition, referring to FIG. 4A, even when the frequency of the TE signal is within the ideal frequency range 138, two low frequency ranges 142 and 144 occur in the TE signal every one 360° rotation of the optical disc 112 because of eccentricity of the optical disc 112.
FIG. 4B shows the optical disc 112 with concentric tracks and a path 146 of positions of the stationary optical pick-up 100 with respect to the optical disc 112 for one 360° rotation of the optical disc 112. Eccentricity of the optical disc 112 means that a center O of the optical disc 112 is offset from a center O′ of rotation of the optical disc 112.
Referring to FIGS. 4A and 4B, the two low frequency ranges 142 and 144 in the TE signal 132 occur around the two points marked X in the path 146 for every one 360° rotation of the optical disc 112. In FIG. 4A, a positive peak value VPP+ of the TE signal 132 is compared with a negative peak value VPP− of the TE signal with respect to the reference voltage, VREF, for offset calibration. However, such a comparison of the peak values VPP+ and VPP− cannot be as easily performed in the prior art with the signal distortion of the TE signal in the two low frequency ranges 142 and 144.
Thus, a mechanism is desired for generating a tracking error signal without delay and without signal distortion from low frequency components.